1. Field of the Invention
The present invention relates to motor controllers and in particular to solid state motor controllers for electric motors adapted for use with portable power equipment, such as lawn and garden equipment.
2. Related Art
Motor controllers are designed to interrupt power to a motor in the event of a fault in the motor. A fault in the motor can cause excessive current to flow through the motor windings and thereby overheat the windings and damage the motor.
To provide overcurrent protection, motor controllers on small motors typically use a relay approach. In the relay approach, when an overcurrent condition is sensed, a circuit breaker connected in series with the motor trips open to interrupt current flow to the motor and halt the motor.
However, circuit breakers do not limit current, they merely interrupt the current path when a particular current threshold has been exceeded for a certain period of time. A problem with using mechanical devices such as circuit breakers to interrupt the current path is that there can be a significant delay between the time the high current condition occurs and the time the circuit breaker trips open. Due to the time delay, high current can flow to the motor as well as to other components of the system before the circuit breaker opens, thereby causing damage to the system. This can be a particular problem with small motors which may be easily damaged by rapid power surges and for which the aprroach may be ineffective.
Another problem with the relay approach is that the circuit breaker is either shut or tripped open. If the current is below a threshold, the circuit breaker remains shut, but if the current exceeds the threshold, the circuit breaker trips open to interrupt the current path. Such an approach is susceptible to spurious trips from transients. If the circuit breaker opens during momentary transients, the operator is forced to reset the circuit breaker before the motor can be restarted thereby causing unnecessary delay and inconvenience.
Also, circuit breaker based protection systems do not readily lend themselves to the addition of auxiliary control features, for example, thermal protection and battery monitoring, in a compact, integrated package.
Therefore, what is needed is a motor controller which can quickly and reliably limit motor current flow and thereby prevent excessively high current through the motor and the system, even for sharp power surges.
What is also needed is a motor controller which can automatically shut down the motor if current limiting is required for a predetermined period of time.
What is also needed is a motor controller which is capable of quickly stopping the motor after an automatic shutdown condition occurs.
What is also needed is a motor controller which provides the flexibility to easily add additional control features for operator interface.
What is also needed is a motor controller which includes the above-cited feature and is adapted for use with portable power equipment, such as lawn and garden equipment.